This invention relates generally to gantry lifts and, more particularly, to header connector plates in such gantry lifts.
Hydraulic gantry lifts are well-known lift devices that are well-suited for certain types of specialized lifts. Known gantry lifts, such as those shown, for example, in U.S. Pat. No. 5,360,123 issued Nov. 1, 1994 to Johnston and commonly owned by the assignee hereof, include a pair of spaced, mobile base units or lift housings, each having an upwardly extendable column. Each column includes a plurality of telescoping sections and a hydraulic cylinder within the sections that operates to raise and lower the column. A horizontal lift beam typically extends across the upper ends of the columns, and the load to be lifted is suspended from the beam. By raising and lowering the columns, the load can be lifted for transport or lowered for disengagement from the beam.
Typically, each of the base units is self-propelled for horizontal movement. After the load is raised, the base units can be actuated to transport the raised load horizontally over the ground, floor or other underlying surface. In the usual arrangement, a track including a pair of spaced, horizontal "I" beams is provided along the path over which the load is to be moved. Each base unit sits on and moves along the beams. By coordinating the extension of the columns and the movement of the base units along the track, the load can be lifted, transported horizontally and then lowered at a new location.
As the load is lifted and transported, it is difficult to maintain precise alignment among the various lift housings, lift columns and lift beam. Slight misalignments can occur if one of the lift columns extends slightly more or less than the other or if one of the lift housings moves slightly ahead of or behind the other along the track. Because of the often considerable length of the lift boom and the resulting lever arm, slight misalignments can nevertheless translate into tremendous torques when multiplied over the length of the lift boom.
To accommodate such unavoidable misalignments and to avoid the potentially destructive forces they can induce, it is common to couple the lift boom to the top of each column through a header plate that permits and accommodates slight movement between the lift boom and lift columns. Typically, the header plate is coupled to the top of the lift column through a structure, such as a half bearing, that permits limited pivoting movement of the header plate relative to the lift column. Half bearings are particularly well suited for this purpose in that they can provide the desired pivoting action while maintaining a low device height or profile. The half bearing, and the header plate that it supports, permit the lift beam to pivot slightly in both the longitudinal and transverse directions relative to the lift column as the gantry lifts and transports the load. This maintains the overall structural integrity of the system while avoiding the development of potentially destructive forces as slight misalignments unavoidably occur.
By nature of their design, half bearings are capable of withstanding large loads in one direction only. When used between the header plate and the top of a lift column, a half bearing has the capacity to bear the weight of the lifted load, but is unable, by itself, to resist forces tending to lift the header plate away from the top of the lift column. Accordingly, the header plate and the half bearing are typically secured to the top of the lift column by means of a bolt passing through the header plate and half bearing. To permit limited pivoting movement of the half bearing, a spacer arrangement is used that provides clearance between the bolt head and a radial or central portion of the half bearing. Although the resulting gap between the underside of the bolt head and the radial part of the half bearing allows the half bearing to pivot, the gap also permits some upward vertical movement of the header plate relative to the top of the lifting column and limits the structural strength of the gantry if a strong separating force is somehow developed.